Humeral Head Arthroplasty
Disease and injury often require shoulder joint arthroplasty using a humeral head prosthesis. There are two types of humeral head prostheses in general use; one is a humeral head resurfacing implant and the other a humeral head replacement implant. Humeral head resurfacing is a conservative approach to humeral head arthroplasty and is usually accomplished by using a thin-wall, dome-shaped shell to resurface the humeral head. The resurfacing implant has a central stem that is placed in the humeral neck following bone preparation to achieve fixation to the humeral bone. In humeral head replacement, the entire humeral head is cut off during surgery, and the humeral head replacement implant has a long medullary stem to fix the implant to the humeral bone.
The shoulder joint is formed by the head of the humerus articulating with a shallow socket called the glenoid. The glenoid is located on the lateral margin of the scapula. Humeral head resurfacing and replacement implants can be total joint implants or hemi joint implants. Total joint implants typically have a polyethylene socket component that replaces the glenoid and articulates with a humeral head replacement.
More than half of the shoulder joint arthroplasty presently done in the US are hemi-arthroplasty because exposure to provide access to the glenoid is difficult. The bone mass comprising the glenoid is limited, and because of this, glenoid replacement components often loosen. For hemi-arthroplasty, humeral head replacement may be presently more commonly used than humeral head resurfacing.
A humeral head resurfacing implant was developed by Dr. S. A. Copeland and was first used clinically in about 1986. The Copeland implant consists of a thin-wall, metal, spherical, dome-shaped shell having a central tapered and fluted stem intended to achieve fixation of the implant to bone.
Humeral head resurfacing arthroplasty has the following benefits when compared to humeral head replacement.
1. Humeral head resurfacing is a conservative procedure requiring much less removal of bone than humeral head replacement. In humeral head replacement, the entire humeral head is removed, and a substantial amount of bone is also removed to make room for the long medullary stem that extends deep into the proximal shaft of the humerus. In humeral head resurfacing, only articular cartilage and a small amount of the subchondral bone is removed from the humeral head to reshape it for reception of the resurfacing implant. In like manner, only a small amount of bone is removed from the humeral head to make room for the implant stem. Minimal bone removal is an objective and a benefit of humeral head resurfacing. The humeral head is left essentially intact during the resurfacing procedure; such maintains the integrity and strength of the humeral head bone structure. Maintaining integrity and strength of the humeral head bone is important because the native humeral head structure provides the foundation needed to support the biomechanical loads that will be encountered by the humeral head resurfacing implant.
2. The orientation of the humeral head with respect to the long axis of the humerus varies considerably from individual to individual. With humeral head replacement, many combinations of head and stem components having different shapes are required to achieve the correct anatomic position of the humeral head for each individual patient. With humeral head resurfacing the position and location of the humeral head is not altered during surgery and the individual anatomy of each patient is preserved.
3. Substantial intra-medullary reaming is not required. Therefore, this is a less traumatic procedure in an elderly patient that reduces risk of fat embolus or hypotension.
4. If there is malunion (non-healed fracture) at the proximal end of the humeral with secondary osteoarthritis, the malunion can be left undisturbed and just the humeral articular surface replaced.
5. With humeral head resurfacing, there is no stem extending down the humeral shaft, and therefore no possibility of humeral shaft bone loss due to stress shielding or a stress riser effect that could result in a low fracture at the tip of the prosthesis.
Humeral Head Resurfacing Surgical Procedure
The surgical procedure consists of making a surgical incision that provides access to the glenohumeral joint so that the shoulder can be dislocated and the humeral head exposed. Once exposed, the size and shape of the humeral head can be determined and an appropriate size implant selected. A guide pin is then placed into the humeral head which serves to orient a cutting instrument that reshapes the humeral head to conform to the concave inner surface of the implant. Following reshaping of the humeral head, a cannulated cutting tool is place over the guide pin and used to form the cavity into which the implant stem will be placed. The resurfacing implant is then placed onto the previously prepared humeral head, and an impactor is then used to firmly seat the implant in bone resulting in a press fit. A radiogragh is then taken to confirm the implant is properly placed.
Contact of the inner concave surface of the dome-shaped implant head portion with the surgically prepared convex mating bone surface of the humeral head provides a large load-bearing area to support joint contact loads. Because the contact between the concave inner surface of the implant and the convex outer surface of the humeral head will not resist rotation of the implant along multiple axes, the implant generally relies upon its stem to resist rotation.
Regarding the implant stem, there are two design options. One is a mono-body configuration, i.e. a single unit where the stem is an integral part of the implant; the other is a modular configuration where stem components of various sizes can be attached to shell components of various sizes. Modular designs are often used for orthopedic joint replacements as a means to accommodate variations in anatomy from one individual to another. However, a modular design necessarily requires a secure means of attaching the stem component to the head component of the device, which is most often accomplished using a taper locking system (e.g. cone-in-cone Morse taper). A locking taper inherently requires that additional material to be used to form the implant stem or inner portion of the shell to form the female component of the cone-in-cone connection. The structure necessary for the female portion of the locking taper takes up additional space, and it requires more bone to be removed as compared to a similar mono-body design. Moreover, the need for removal of the additional bone required for such a taper lock modular design violates the objective of the resurfacing design principle, namely minimal bone removal, and as a result, it reduces the load-bearing capacity of the surgically modified humeral head. Thus, for a humeral head resurfacing implant, a one-piece mono-body configuration that will require substantially less bone removal should be the preferred design.
Humeral Head Resurfacing Implant Fixation
Long Term Fixation
Fixation of the humeral head resurfacing implant to bone can be achieved using bone cement or by means of material capable of achieving biological fixation. Bone cement is known to cause chemical and thermal bone damage during insertion resulting bone necrosis and is known to fracture and fragment while in situ. Both of these factors can result in loss of implant fixation. Biological fixation, where living bone attaches permanently to the implant surface, is considered an advantageous alternative to bone-cement fixation. Biocompatible materials, such as titanium, that allow direct bone to implant adaptation resulting in osseous integration, and porous material coatings that allow for bone ingrowth and hydroxyapatite (HA) coatings that result in a bone to HA bond are means of achieving biological attachment. Biological attachment relies on the bone's natural healing ability to achieve fixation of the implant.
Primary Fixation
Achieving long term stable biological fixation of implant requires time for the bone healing process to integrate, grow into or bond bone to the implant stem. In this regard, achieving biological fixation of implant to bone is similar in principle to the healing of a fractured bone. Following a fracture, a biologic response generates new bone to bridge the fracture and unite the pieces of the fractured bone. During the fracture healing process, it is necessary that the ends of the fractured bone are immobile. Immobilizing fracture bones is accomplished clinically by applying an external cast or using internal fixation devices such a plates and screws, wires or intramedullary rods. If the fracture fragments are not adequately immobilized during the 6-8 weeks necessary for fracture healing, it is likely the fracture will not heal, resulting in a non-union (malunion). A requirement for 6-8 weeks of immobilization to achieve fracture healing following surgery also applies to achieving biological fixation of an implant. The implant must be immobile to allow the bone tissue to integrate, grow into and or bond to the implant stem. If the implant is not immobile during the post-operative healing period it is likely a secure biological attachment of implant to bone will not be achieved.
In the case of a humeral head resurfacing implant it is the stem of the implant that provides the primary fixation required to achieve biological attachment. The implant stem must be designed to provide adequate post-operative immobilization for a period of 6 8 weeks so that biological fixation of the implant stem to bone can be achieved.
Humeral Head Resurfacing Implants in Current Use
Two humeral head resurfacing devices are in common use at this time, one produced by Biomet Orthopedics (Copeland implant) and the other produced by DePuy Orthopedics (Global C. A. P. implant).
The Biomet Copeland humeral head resurfacing implant is a mono-body device consisting of a dome-shaped shell having a spherical convex outer articular surface, a concave inner surface and a central peg shaped stem to achieve fixation in the humeral bone. The device is made of ASTM F-75 Co—Cr casting alloy, and the outer convex surface is polished and intended to act as the articulating surface. The inner concave surface is intended to bear against the surgically prepared humeral head. A tapered, four fluted stem extends outward from the center on the inner concave surface of the shell. The stem is inserted into a surgically created cavity made in the humeral head and is intended to stabilize the device in the humeral bone. The inner concave surface of the dome-shaped shell has a plasma-sprayed titanium layer to achieve osseous integration and is available with a plasma-sprayed hydroxyapatite (HA) layer placed on the titanium layer to promote bonding of the implant to bone. The Copeland Humeral Head Resurfacing Implant is approved for use with and without bone-cement.
The DePuy C.A.P. humeral head resurfacing implant is a mono-body device consisting of a dome-shaped shell having a spherical convex outer articular surface, a concave inner surface and a central peg shaped stem to achieve fixation in the humeral bone. The device is made of ASTM F-75 Co—Cr casting alloy, and the outer convex dome surface is polished to act as the articulating surface. The inner concave dome surface has a porous Co—Cr alloy layer intended to bear against the surgically prepared humeral head. The tapered stem has a frusto-conical upper section that extends outward from the center of the inner concave surface of the dome and a cruciate lower section. The innermost surface of the concave dome where the stem connects to the dome is flat; that is, the concave portion of the dome has the shape of a truncated sphere. A porous coating on the stem extends approximately one half-way down the stem of the implant. The distal portion of the stem has four flutes providing rotational stability to the implant. The stem is inserted into a surgically created cavity made in the humeral head and is intended to stabilize the device in the humeral bone. As an added feature to enhance fixation, the C.A.P. implant can be obtained with a hydroxyapatite (HA) coating placed on the porous Co—Cr layer. The DePuy C.A.P. Humeral Head Resurfacing Implant is approved for use with and without bone-cement.
Despite the fact that there are various humeral head resurfacing implants on the market in the United States at the present time, none of them is considered to be totally satisfactory. Other examples of humeral head resurfacing implants are found in U.S. Pat. Nos. 6,520,964; 6,783,549; and 7,517,364 and in Published Application Nos. 2006/0009852, 2007/0156250, 2007/0225822 and 2008/0021564. The humeral head components of these implants (both for humeral head replacement and resurfacing) in commercial use today in total and hemi joint replacement are generally made of Co—Cr alloy. It is recognized that Co—Cr alloy is damaging to joint tissues (cartilage and bone), and this is a shortcoming of such hemi-arthroplasty devices. However, from an overall standpoint, Co—Cr alloy has become the material of present choice. Pyrolytic carbon (pyrocarbon) has been shown to be much less damaging to native joint tissues (cartilage and bone); thus, it would be a better material for hemi-arthroplasty than either metal or ceramics such as aluminum oxide or zirconia. However, pyrolytic carbon has significantly different properties, and as a result has generally achieved commercial use primarily on articular surfaces.
Accordingly, improvements in such resurfacing implants continue to be sought, particularly ones that would utilize pyrocarbon.